The formation and development of images on the imaging surfaces of photoconductive materials by electrostatic means is well-known (Carlson, U.S. Pat. No. 2,297,691). The best known of the commercial processes, more commonly known as xerography, forms a latent electrostatic image on the surface of an imaging layer by uniformly electrostatically charging the surface in the dark, and then exposing the charged surface to a light and shadow image. The light-struck areas of the imaging layer are thus made substantially more charge-conductive and the electrostatic charge is selectively dissipated in such areas. After light exposure, the latent electrostatic image remaining on the imaging surface (i.e. a positive electrostatic image) is made visible by contacting with finely divided colored or black electroscopic material, known in the art as "toner". Toner is principally attracted to those areas on the image bearing surface which retain the original electrostatic charge and thereby form a visible positive image.
In structure, the conventional xerographic plate normally has a photoconductive insulating layer overlaying the conductive base or substrate and frequently an interface or charge blocking layer between the two.
The photoconductive layer may comprise a number of materials known in the art. For example, selenium-containing photoconductive material such as vitreous selenium, or selenium modified with varying amounts of arsenic are found very useful in commercial xerography. Generally speaking, the photoconductive layer should have a specific resistivity greater than about 10.sup.10 ohm-cm (preferably 10.sup.13 ohm-cm) in the absence of illumination. In addition, resistivity should drop at least several orders of magnitude in the presence of an activating energy source such as light. As a practical matter, a photoconductive layer should support an electrical potential of at least about 100 volts in the absence of light or other actinic radiation, and may usefully vary in thickness from about 10 to 200 microns.
In addition to the above, photoconductive layers will also normally exhibit some reduction in potential or voltage leak, even in the absence of an activating light. This phenomenon, known as "dark decay", will vary somewhat with the amount of usage of the photoreceptor. The existence of this problem is well-known and has been controlled, where necessary, by incorporation of an interface or barrier layer such as a very thin dielectric film or layer between the substrate and the photoconductive insulating layer. U.S. Pat. No. 2,901,348 to Dessauer et al. utilizes a layer of aluminum oxide in this manner. With some limitations, blocking interface layers can effectively function not only to permit a photoconductive layer to support a charge of relatively high field strength, and to substantially minimize dissipation (dark decay) in the absence of illumination, but also to aid in cementing the photoconductive layer to the substrate. When activated by illumination, however, the interface-photoconductor layer combination must still be sufficiently conductive to permit dissipation or discharge of a substantial portion of the applied charge through the photoconductive layer.
When the newer, more sensitive inorganic selenium alloys such as arsenic-rich selenium alloys (ref. U.S. Pat. Nos. 2,822,300, 2,802,542, and 3,312,548) are utilized as photoconductors, it is particularly important for obtaining proper charging characteristics and minimal dark decay that the photoreceptor be properly "broken in" or "aged" before regular usage.
Normally, the breaking in procedure for a photoreceptor of the selenium type can require dark storage for an extended period followed by a run of up to about a thousand test copies. For various reasons, however, the break in period must sometimes be carried out without adequate control over all of the physical parameters, such as temperature, humidity and general atmospheric contamination. Since such parameters have a substantial effect on the ultimate life and efficiency of the photoreceptor, it is very desirable that the breaking in process be of short duration and avoid as many uncontrolled variables as possible.
An equally troublesome problem with the newer, faster Se-As-type photoconductors is a tendency to sacrifice low density copy resolution in favor of speed and increased range of light sensitivity.
It is an object of the present invention to improve the operating efficiency of selenium and arsenic-containing photoreceptors of the xerographic type to obtain improved resolution from low density copies.